1. Field of the Invention
The present invention generally relates to superconducting junction elements and superconducting junction circuits, and more particularly to a superconducting junction element and a superconducting junction circuit which can control a critical current density or a critical current.
2. Description of the Related Art
A critical temperature at which the phase changes from a normally conducting state to a superconducting state was 23K or less for a metal alloy. However, since the copper oxide superconductor YBa2Cu3O7-x (YBCO) having a critical temperature of 92K was found in 1987, active research has been made on oxide superconductors. In the case of the oxide superconductors, the critical temperature at which the phase changes from the normally conducting state to the superconducting state is higher than that of liquid nitrogen. In addition, the cooling cost of the oxide superconductors can be reduced considerably compared to liquid He because the oxide superconductors can use a cooler or liquid nitrogen as the cool medium. Furthermore, when a magnetic field is applied to the oxide superconductors, a decrease in the critical current is considerably small compared to that of the conventional metal superconductors.
It is expected that such oxide superconductors will be used in various fields. For example, a superconducting junction circuit having a superconducting junction element may be used in the field of superconductor electronics, as may be seen from Katsuno et al., “A Novel Multilayer Process for HTS SFQ Circuit”, IEEE Transactions on Applied Superconductivity, Vol. 13, No. 2, pp. 809-812, June 2003. The superconducting junction element has two superconductors coupled via a superconducting junction part having weak superconductivity. For example, the flux quantum of the superconducting junction element, that is peculiar to the superconducting phenomenon, is controllable by an external voltage that is applied to the superconducting junction element. Hence, the superconducting junction circuit has high-speed response and low-noise characteristics due to the macroscopic quantum effect.
FIG. 1 is an equivalent circuit diagram of a conventional superconducting junction circuit. A description will be given of an operation of a superconducting junction circuit 200 shown in FIG. 1 that is formed by two superconducting junction elements 201a and 201b. The superconducting junction element 201a has a superconducting junction 202a, and the superconducting junction element 201b has a superconducting junction 202b. A D.C. current (or bias current) Ib from a D.C. current source 23 is supplied to the superconducting junction 202a via one of the superconductors thereof, and a D.C. current from another D.C. current source 23 is supplied to the superconducting junction 202b via one of the superconductors thereof. In addition when a current pulse caused by a flux quantum ø0 is supplied to the input end, this current pulse is superimposed on the bias current Ib flowing through the superconducting junction 202a, and the current flowing through the superconducting junction 202a increases. In this state, the superconducting junction 202a switches from a “zero voltage state A” which is a superconducting state to a “finite voltage state B” as shown in FIG. 2 because the current flowing through the superconducting junction 202a exceeds a critical current Ic. FIG. 2 is a diagram for explaining the operation of the conventional superconducting junction circuit 200. In FIG. 2, the ordinate indicates the current I in arbitrary units, and the abscissa indicates the voltage V in arbitrary units. Furthermore, a current pulse caused by the flux quantum is supplied to the superconducting junction 202b due to the switching of the superconducting junction 202a to the “finite voltage state B”, and the superconducting junction 202b similarly switches from the “zero voltage state A” to the “finite voltage state B”. Accordingly, the flux quantum successively propagates through the superconducting junction elements 201a and 201b. Since the switching of the superconducting junctions 202a and 202b is based on the tunneling phenomenon, the switching time is 1 psec or less. Moreover, it is estimated that the power consumption of each of the superconducting junction elements 201a and 201b is several nW because the superconducting junction elements 201a and 201b operate at an extremely low current level, and it is expected that ultra high-speed switching elements having a low power consumption can be realized thereby.
FIG. 3 is a plan view showing the superconducting junction circuit corresponding to the equivalent circuit diagram shown in FIG. 1, and FIG. 4 is a cross sectional view taken along a line A-A in FIG. 3. In FIG. 3, the illustration of the D.C. current sources is omitted.
As shown in FIGS. 3 and 4, the superconducting junction element 201a of the superconducting junction circuit 200 has a lower electrode 206 provided on a substrate 205, a barrier layer 208 provided on an end surface of the lower electrode 206, an upper electrode 209 contacting the barrier layer 208, an insulator layer 210 insulating the upper and lower electrodes 209 and 206, and a protection layer 211 covering the upper electrode 209. The superconducting junction element 201b has a structure similar to that of the superconducting junction element 201a. The upper electrodes 209 of the two superconducting junction elements 201a and 201b are mutually connected, and the barrier layers 208 form the superconducting junctions 202a and 202b. The bias currents from the D.C. current sources 203 (not shown in FIGS. 3 and 4) flow to the lower electrodes 206 of the superconducting junction elements 201a and 201b via the upper electrodes 209 and the barrier layers 208. The two lower electrodes 206 are grounded.
It is known that the critical current of each of the superconducting junctions 202a and 202b of the superconducting junction circuit 200 is dependent on the junction area of the corresponding barrier layer 208. If the thickness of the lower electrode 206 and the inclination angle of the barrier layer 208 of the superconducting junction element 201a are approximately the same as the thickness of the lower electrode 206 and the inclination angle of the barrier layer 208 of the superconducting junction element 201b, the junction area of each of the barrier layers 208 depends on a width JWX of the corresponding upper electrode 209 shown in FIG. 3. Hence, by setting the widths of the upper electrodes 209 approximately the same, the critical currents of the superconducting junctions 202a and 202b will become approximately the same, and a stable operation of the superconducting junction circuit 200 will be obtained in this case.
However, in the case of the superconducting junction circuit 200 shown in FIGS. 3 and 4, the actual critical currents (corresponding to Ic in FIG. 2) of the superconducting junction elements 201a and 201b are inconsistent. For this reason, even if a current pulse caused by the flux quantum is supplied to the superconducting junction elements, the switching may not occur in some superconducting junction elements or, the current margin may be narrow in some superconducting junction elements even if the switching does occur. As a result, there is a problem in that a basic logic circuit or superconducting transmission line (or Josephson transmission line) that is formed by a small number of superconducting junction elements will not operate stably. In addition, this problem becomes more conspicuous and serious as the number of superconducting junction elements forming the basic logic circuit or superconducting transmission line increases.